When communications are transmitted in a synchronous digital telecommunications system, for example a SDH (Synchronous Digital Hierarchy) system or a SONET (Synchronous Optical Network) system, what essentially matters is that all network elements (e.g. add/drop multiplexers, cross-connects or line multiplexers) operate synchronously with each other. An article by R. Kiefer entitled "Measurement tasks on SDH transmission systems", 1993 ntz volume 46, issue 2, pages 92-96, describes that network elements are synchronized by means of a reference clock (designated as a 2 MHz central clock). It further describes that to simulate non-ideal reference clock responses, an analyzer can be switched between the reference clock source and the network element, to detune certain steps of the reference clock frequency. The network element reacts with pointer operations, which are documented by the analyzer to identify weak areas in the system's response. If the reference clock deviates too much, the network element switches over to an alternative synchronization.
However, the described method does not allow testing the connections through which a network element is provided with the reference clock, and with this method it is not possible to determine through which connections a network element receives the reference clock that is used for the synchronization.